To characterize those functions specific to Tle4, we developed a novel Tle4 null mouse model. Our studies have identified the critical importance of Tle4 in bone calcification, bone marrow niche formation, BM cellularity, B cell development, HSPC self-renewal capacity, and thymic and splenic architecture. The Groucho/TLE proteins are capable of interacting with multiple signaling pathways and may alter the function of key proteins important for bone formation, including Runx2/Cbfa1, a critical regulator of bone development and maturation. The apparent resorption of trabeculae seen in 4 week old mice under the growth plate suggests additional abnormalities in addition to bone formation. Further investigations are underway to Fulvestrant citations better characterize the nature of this defect in bone development and maintenance. Several reports have described the role of the BM microenvironment on hematopoiesis. Osteoblasts have a welldefined role in supporting B lymphopoiesis via expression of the heterotrimeric G protein alpha subunit Galpha. While we have not yet determined whether osteoblasts are specifically affected in our model per se, it is reasonable to assume that the compromise of trabecular bone is at least contributory to the observed B cell and HSPC defects. The inability of Tle4 null stromal cells cultured in vitro to maintain WT HSC suggests that the hematopoietic phenotype seen in KO mice may derive in part from niche-induced deregulation. Furthermore, as evidenced by TUNEL staining of bones harvested from WT and KO mice, it is clear that the absence of Tle4 has an effect on the viability and integrity of the BM niche and stroma, Further experiments are needed to better characterize the nature of this defective stromal support of HSPC. Concurrently, BM and fetal liver serial transplantation experiments demonstrate a robust HSPC-intrinsic effect of Tle4 deletion. In both transplant models, mice receiving KO HSPC develop peripheral leukopenia. Moreover, this finding in FL serial transplantation illustrates the potential HSPC-intrinsic defects of Tle4 loss leading to decreased capacity of HSPC self-renewal. Additionally, our study provides the first direct in vivo evidence of a role of Tle4 on B-cell development, an effect previously inferred based on interactions of Tle4 and Pax5. The somewhat distinct block in B-cell differentiation seen with Tle4 loss compared to that reported with Pax5 loss suggests Tle4 may exert some B-cell effects independent from Pax5, although we can’t exclude potential animal models differences as accounting for this effect. Taken together, our data demonstrates the critical importance of Tle4 in regulating various developmental processes central to bone maturation, medullary hematopoiesis, and HSPC maintenance. These findings may have significant implications for understanding hematopoiesis in both normal and disease states. Moreover, our observations provide further insight and affirmation to previous findings that implicate Tle4 as a critical regulator of leukemia and other states of hematological dysregulation. In summary, by the development of the first model for Tle4 deletion in mammals, our data provide evidence for an essential role for Tle4 in mammalian bone and blood development.